Prosocial behavior can improve psychological well-being and physical health. However, the underlying biological mechanisms that mediate the relationship between prosociality and health remain unclear. In this pre-registered experiment, we tested whether a 4-week kindness intervention could slow leukocyte telomere shortening and increase well-being. Community adults (N ​= ​230) were randomly assigned to complete 1 of 3 activities, each week for 4 weeks: to perform 3 kind acts for other people, to perform 3 kind acts for themselves, or to list daily activities. At baseline and post-intervention, participants came to the lab to provide a small dried blood spot (DBS) sample via finger prick for analysis of telomere length. Participants completed psychological measures (e.g., loneliness, life satisfaction) at baseline, post-intervention, and at the 2-week follow up. Participants who performed kind acts for others did not demonstrate hypothesized changes in well-being, nor in telomere length, relative to controls. Exploratory analyses revealed that, relative to controls, participants who did kind acts for others showed reductions in loneliness through the 2-week follow up. The salubrious effects of prosocial behavior in the short term are not likely due to the inhibition of cellular aging (at least as indexed by telomere length). However, extending kindness to others holds promise as a future research direction for interventions to alleviate loneliness. •Prosocial behavior is associated with better health, but mechanisms remain unclear.•We report a pre-registered investigation of prosocial behavior and telomere length.•A 4-week prosocial behavior intervention did not slow rates of telomere shortening.•Performing prosocial behavior for others was linked with reductions in loneliness.•Prosocial behavior may reduce loneliness but does not appear to impact telomeres.